It is desirable for digitally controlled oscillators (DCOs) to have a stable output frequency. However, it is known for the output frequency of DCOs to vary with temperature. DCOs that are based on ring oscillator topologies tend to exhibit more temperature sensitivity than those that are based on an LC oscillator. Even LC oscillator based DCOs (LC-DCOs) may have too much temperature sensitivity.
It is known to try to correct temperature variation in the output frequency from a DCO. One method is to use a temperature sensor and a temperature compensation capacitor bank. This approach can improve temperature sensitivity, but temperature sensor inaccuracies and temperature dependence of analog varactors has previously made very low temperature drift (e.g. <500 ppm/K) difficult or impossible to achieve.
An improved method for stabilising the frequency of a digitally controlled oscillator in response to temperature variation is desirable.